They have a duopoly, so all app developers are extorted to use this if they want to make a living. (Or they use the duopoly-sanctioned payment methods and pay the extortion money there)
You aren't forced to add links to external content in your app. It was already prohibited, so it's not like you have to do extra work in removing them.
Making the process exhorbitantly difficult and expensive is obviously intended to make sure people choose Google's preferred path. It's a bog standard monopoly play.
